166 On the Influence of the Humidity of the Atmosphere 
Having occasion, during the month of August 1822, to fix 
my residence for some time at Amulree a situation in the 
Highlands of Perthshire remarkable for the dryness and salu- 
brity of its air, I carried along with me a number of good 
meteorological instruments, for the purpose of making such ob- 
servations on the hygrometric state of the air, in that elevated 
part of the country, as might tend either to confirm or overturn 
my hypothesis. The state of the weather, which proved ex- 
tremely variable, was highly favourable to the object I had in 
view, the thermometer having ranged, in that short interval, from 
63° to 26°, and exhibited greater fluctuations than I have ever 
observed in the month of August. On the 13th the day was 
hazy, and the coincidence, the following night, between the 
minimum temperature and the point of deposition, as indicated 
by my formula, was exact. The following day the humidity of 
the air increased considerably, and the point of deposition rose 
from 45i° to 48J°. The ensuing night the minimum tempera- 
ture was proportionally raised, the register-thermometer having 
indicated 50°. The next day, the absolute humidity of the air 
was reduced, in consequence of an impetuous dry wind, from 
.282 to .165 grains of moisture in 100 cubic inches of air, and 
the point of deposition thus descended to 37|°. The minimum 
temperature which followed this very sudden change was 385°, 
differing only |ths of a degree from the temperature assigned 
by the formula. From that day the dryness of the atmosphere 
gradually increased till the 22d, when the quantity of moisture 
in the air became less than is'commonly observed, except in the 
most rigorous of the winter months. On the evening of that day 
my formula indicated that the point of deposition was reduced 
to 26°, and the next morning I had the satisfaction to find that 
the register-thermometer had, in the course of the night, reached 
the very same point. Another register-thermometer, which I 
had placed on the summit of a neighbouring hill, at the height 
of 905 feet above the point where I usually made my observa- 
tions, shewed that the greatest cold in that elevated situation 
had, contrary to what commonly happens, been 1° higher. 
* Amulree is, by my barometrical observations, 935 feet above the level of 
the sea. 
